Abstract
Abstract Continued follow-up of WISEA J153429.75−104303.3, announced in Meisner et al., has proven it to have an unusual set of properties. New imaging data from Keck/MOSFIRE and HST/WFC3 shows that this object is one of the few faint proper motion sources known with J − ch2 >8 mag, indicating a very cold temperature consistent with the latest known Y dwarfs. Despite this, it has W1−W2 and ch1−ch2 colors ∼1.6 mag bluer than a typical Y dwarf. A new trigonometric parallax measurement from a combination of WISE, Spitzer, and HST astrometry confirms a nearby distance of 16.3 − 1.2 + 1.4 pc and a large transverse velocity of 207.4 ± 15.9 km s−1. The absolute J, W2, and ch2 magnitudes are in line with the coldest known Y dwarfs, despite the highly discrepant W1−W2 and ch1−ch2 colors. We explore possible reasons for the unique traits of this object and conclude that it is most likely an old, metal-poor brown dwarf and possibly the first Y subdwarf. Given that the object has an HST F110W magnitude of 24.7 mag, broadband spectroscopy and photometry from JWST are the best options for testing this hypothesis.
Highlights
The coldest brown dwarfs are a difficult population to characterize
Using synthetic photometry on 13 high-S/N G102+G141 Hubble Space Telescope (HST) spectra of late-T and -Y dwarfs from Schneider et al (2015), we find that F110W magnitudes are fainter than J magnitudes by 0.79 mag
We have better constrained the J − ch2 color, measured a robust parallax, and derived absolute magnitudes for Wide-Field Infrared Survey Explorer (WISE) 1534−1043. These results were obtained through new J-band imaging from Keck that fails to detect the object and even deeper F110W imaging from HST that reveals the source at S/ N ≈ 5
Summary
The coldest brown dwarfs are a difficult population to characterize. What little flux they emit is concentrated near 5 μm, and, because this region is largely unobservable from the ground, space-based missions offer the best chance of discovery and follow-up. For known Y dwarfs—the coldest brown dwarfs with effective temperatures below ∼450K (Cushing et al 2011)—absolute J-band magnitudes range from 19.4 to 28.2 mag (Kirkpatrick et al 2021). Despite estimates that 15-+1210 such frigid objects (Wright et al 2014) have been imaged in data sets by the Wide-Field Infrared Survey Explorer (WISE; Wright et al 2010; Mainzer et al 2014), it is the only brown dwarf recognized to have Teff < 350 K (Kirkpatrick et al 2021), two others may fall in this temperature zone—WISEA J083011.95+283716.0 (hereafter, WISE 0830+2837; Bardalez Gagliuffi et al 2020) and WISEPA J182831.08+265037.8 (hereafter, WISE 1828+2650; Cushing et al 2011). New data provide a robust parallax and extremely red J − ch color The parallax places it securely within 20 pc, and the colors and absolute magnitudes are unlike those of any brown dwarf known. WISE 1534−1043 appears to be the most enigmatic cold brown dwarf yet identified and bolsters the idea that such objects span a wide range of observational properties
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